snapshot: framework for more efficient relation traversal

No one was using virDomainSnapshotHasChildren, but that was an
O(n) function.  Exposing and tracking a bit more metadata for each
snapshot will allow the same query to be made with an O(1) query
of the member field.  For single snapshot operations (create,
delete), callers can be trusted to maintain the metadata themselves,
but for reloading, we can't compute parents as we go since there
is no guarantee that parents were parsed before children, so we also
provide a function to refresh the relationships, and which can
be used to detect if the user has ignored our warnings and been
directly modifying files in /var/lib/libvirt/qemu/snapshot.  This
patch only adds metadata; later patches will actually use it.

This layout intentionally hardcodes the size of each snapshot struct,
by tracking sibling pointers, rather than having to deal with the
headache of yet more memory management by directly sticking a
dynamically sized child[] on each parent.

* src/conf/domain_conf.h (_virDomainSnapshotObj)
(_virDomainSnapshotObjList): Add members.
(virDomainSnapshotUpdateRelations, virDomainSnapshotDropParent):
New prototypes.
(virDomainSnapshotHasChildren): Delete.
* src/conf/domain_conf.c (virDomainSnapshotSetRelations)
(virDomainSnapshotUpdateRelations, virDomainSnapshotDropParent):
New functions.
(virDomainSnapshotHasChildren): Drop unused function.
* src/libvirt_private.syms (domain_conf): Update exports.

src/conf/domain_conf.c

@@ -12312,13 +12312,6 @@ virDomainSnapshotForEachChild(virDomainSnapshotObjListPtr snapshots,
 
     return act.number;
 }
-
-int virDomainSnapshotHasChildren(virDomainSnapshotObjPtr snap,
-                                 virDomainSnapshotObjListPtr snapshots)
-{
-    return virDomainSnapshotForEachChild(snapshots, snap, NULL, NULL);
-}
-
 typedef enum {
     MARK_NONE,       /* No relation determined yet */
     MARK_DESCENDANT, /* Descendant of target */
@@ -12423,6 +12416,111 @@ virDomainSnapshotForEachDescendant(virDomainSnapshotObjListPtr snapshots,
     return act.number;
 }
 
+/* Struct and callback function used as a hash table callback; each call
+ * inspects the pre-existing snapshot->def->parent field, and adjusts
+ * the snapshot->parent field as well as the parent's child fields to
+ * wire up the hierarchical relations for the given snapshot.  The error
+ * indicator gets set if a parent is missing or a requested parent would
+ * cause a circular parent chain.  */
+struct snapshot_set_relation {
+    virDomainSnapshotObjListPtr snapshots;
+    int err;
+};
+static void
+virDomainSnapshotSetRelations(void *payload,
+                              const void *name ATTRIBUTE_UNUSED,
+                              void *data)
+{
+    virDomainSnapshotObjPtr obj = payload;
+    struct snapshot_set_relation *curr = data;
+    virDomainSnapshotObjPtr tmp;
+
+    if (obj->def->parent) {
+        obj->parent = virDomainSnapshotFindByName(curr->snapshots,
+                                                  obj->def->parent);
+        if (!obj->parent) {
+            curr->err = -1;
+            VIR_WARN("snapshot %s lacks parent", obj->def->name);
+        } else {
+            tmp = obj->parent;
+            while (tmp) {
+                if (tmp == obj) {
+                    curr->err = -1;
+                    obj->parent = NULL;
+                    VIR_WARN("snapshot %s in circular chain", obj->def->name);
+                    break;
+                }
+                tmp = tmp->parent;
+            }
+            if (!tmp) {
+                obj->parent->nchildren++;
+                obj->sibling = obj->parent->first_child;
+                obj->parent->first_child = obj;
+            }
+        }
+    } else {
+        curr->snapshots->nroots++;
+        obj->sibling = curr->snapshots->first_root;
+        curr->snapshots->first_root = obj;
+    }
+}
+
+/* Populate parent link and child count of all snapshots, with all
+ * relations starting as 0/NULL.  Return 0 on success, -1 if a parent
+ * is missing or if a circular relationship was requested.  */
+int
+virDomainSnapshotUpdateRelations(virDomainSnapshotObjListPtr snapshots)
+{
+    struct snapshot_set_relation act = { snapshots, 0 };
+
+    virHashForEach(snapshots->objs, virDomainSnapshotSetRelations, &act);
+    return act.err;
+}
+
+/* Prepare to reparent or delete snapshot, by removing it from its
+ * current listed parent.  Note that when bulk removing all children
+ * of a parent, it is faster to just 0 the count rather than calling
+ * this function on each child.  */
+void
+virDomainSnapshotDropParent(virDomainSnapshotObjListPtr snapshots,
+                            virDomainSnapshotObjPtr snapshot)
+{
+    virDomainSnapshotObjPtr prev = NULL;
+    virDomainSnapshotObjPtr curr = NULL;
+    size_t *count;
+    virDomainSnapshotObjPtr *first;
+
+    if (snapshot->parent) {
+        count = &snapshot->parent->nchildren;
+        first = &snapshot->parent->first_child;
+    } else {
+        count = &snapshots->nroots;
+        first = &snapshots->first_root;
+    }
+
+    if (!*count || !*first) {
+        VIR_WARN("inconsistent snapshot relations");
+        return;
+    }
+    (*count)--;
+    curr = *first;
+    while (curr != snapshot) {
+        if (!curr) {
+            VIR_WARN("inconsistent snapshot relations");
+            return;
+        }
+        prev = curr;
+        curr = curr->sibling;
+    }
+    if (prev)
+        prev->sibling = snapshot->sibling;
+    else
+        *first = snapshot->sibling;
+    snapshot->parent = NULL;
+    snapshot->sibling = NULL;
+}
+
+
 int virDomainChrDefForeach(virDomainDefPtr def,
                            bool abortOnError,
                            virDomainChrDefIterator iter,

src/conf/domain_conf.h

@@ -1460,6 +1460,11 @@ typedef virDomainSnapshotObj *virDomainSnapshotObjPtr;
 struct _virDomainSnapshotObj {
     virDomainSnapshotDefPtr def;
 
+    virDomainSnapshotObjPtr parent; /* NULL if root */
+    virDomainSnapshotObjPtr sibling; /* NULL if last child of parent */
+    size_t nchildren;
+    virDomainSnapshotObjPtr first_child; /* NULL if no children */
+
     /* Internal use only */
     int mark; /* Used in identifying descendents. */
 };
@@ -1470,6 +1475,9 @@ struct _virDomainSnapshotObjList {
     /* name string -> virDomainSnapshotObj  mapping
      * for O(1), lockless lookup-by-name */
     virHashTable *objs;
+
+    size_t nroots;
+    virDomainSnapshotObjPtr first_root;
 };
 
 typedef enum {
@@ -1510,8 +1518,6 @@ virDomainSnapshotObjPtr virDomainSnapshotFindByName(const virDomainSnapshotObjLi
                                                     const char *name);
 void virDomainSnapshotObjListRemove(virDomainSnapshotObjListPtr snapshots,
                                     virDomainSnapshotObjPtr snapshot);
-int virDomainSnapshotHasChildren(virDomainSnapshotObjPtr snap,
-                                virDomainSnapshotObjListPtr snapshots);
 int virDomainSnapshotForEachChild(virDomainSnapshotObjListPtr snapshots,
                                   virDomainSnapshotObjPtr snapshot,
                                   virHashIterator iter,
@@ -1520,6 +1526,9 @@ int virDomainSnapshotForEachDescendant(virDomainSnapshotObjListPtr snapshots,
                                        virDomainSnapshotObjPtr snapshot,
                                        virHashIterator iter,
                                        void *data);
+int virDomainSnapshotUpdateRelations(virDomainSnapshotObjListPtr snapshots);
+void virDomainSnapshotDropParent(virDomainSnapshotObjListPtr snapshots,
+                                 virDomainSnapshotObjPtr snapshot);
 
 /* Guest VM runtime state */
 typedef struct _virDomainStateReason virDomainStateReason;

src/libvirt_private.syms

@@ -410,10 +410,10 @@ virDomainSnapshotAssignDef;
 virDomainSnapshotDefFormat;
 virDomainSnapshotDefFree;
 virDomainSnapshotDefParseString;
+virDomainSnapshotDropParent;
 virDomainSnapshotFindByName;
 virDomainSnapshotForEachChild;
 virDomainSnapshotForEachDescendant;
-virDomainSnapshotHasChildren;
 virDomainSnapshotObjListGetNames;
 virDomainSnapshotObjListGetNamesFrom;
 virDomainSnapshotObjListNum;
@@ -421,6 +421,7 @@ virDomainSnapshotObjListNumFrom;
 virDomainSnapshotObjListRemove;
 virDomainSnapshotStateTypeFromString;
 virDomainSnapshotStateTypeToString;
+virDomainSnapshotUpdateRelations;
 virDomainSoundDefFree;
 virDomainSoundModelTypeFromString;
 virDomainSoundModelTypeToString;